Technical Abstract:
Antibiotics are used in food animal production for growth promotion; these drugs typically target Gram-positive microorganisms. An anaerobic continuous-flow fermentation culture of chicken gastrointestinal microorganisms has been used as a model of the chicken ceca to study interactions between normal avian gut microflora and enteropathogens. Previous studies have shown that such a culture clears E. coli O157:H7 at a rate of one log/CFU/day. The purpose of this study was to determine if virginiamycin, an antibiotic used for growth promotion that targets Gram- positive microorganisms and anaerobes, would provide a selective growth advantage for E. coli O157:H7; thus preventing clearance from the continuous-flow culture. Two of three identical chicken continuous-flow cultures were treated with 1.0 ug/ml virginiamycin, a concentration considered to be subtherapeutic. After one week of virginiamycin treatment, 1.0 x 10**7 CFU/ml E. coli O157:H7 was added to all three cultures. For the duration of the experiment the endogenous bacteroides, veillonella, lactic acid bacteria and enterococci in the virginiamycin treated cultures remained at the same concentration as those in the untreated control culture. The E. coli O157:H7 population dropped at a rate of one log/CFU/day until it was undetectable in all three cultures. The results of this study suggest that a subtherapeutic level of 1.0 ug/ml virginiamycin was not sufficient to perturb the microbial ecology of the culture such that E. coli O157:H7 was provided with a competitive advantage.